1. Field of the Invention
The invention is related to non-volatile magnetic storage devices, and in particular, to a hard disk drive including a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) read sensor with smoothened multiple reference layers.
2. Statement of the Problem
Of the many non-volatile magnetic storage devices, a hard disk drive is the most extensively used to store data. The hard disk drive includes a hard disk and an assembly of write and read heads. The assembly of write and read heads is supported by a slider that is mounted on a suspension arm. When the hard disk rotates, an actuator swings the suspension arm to place the slider over selected circular data tracks on the hard disk. The suspension arm biases the slider toward the hard disk, and an air flow generated by the rotation of the hard disk causes the slider to fly on a cushion of air at a very low elevation (fly height) over the hard disk. When the slider rides on the air, the actuator moves the suspension arm to position the write and read heads over selected data tracks on the hard disk. The write and read heads write data to and read data from, respectively, data tracks on the hard disk. Processing circuitry connected to the write and read heads then operates according to a computer program to implement writing and reading functions.
In a reading process, the read head passes over magnetic transitions of a data track on the rotating hard disk, and magnetic fields emitting from the magnetic transitions modulate the resistance of a read sensor in the read head. Changes in the resistance of the read sensor are detected by a sense current passing through the read sensor, and are then converted into voltage changes that generate read signals. The resulting read signals are used to decode data encoded in the magnetic transitions of the data track.
In a typical read head, a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) read sensor is electrically separated by side oxide layers from longitudinal bias layers in two side regions for preventing a sense current from shunting into the two side regions, but is electrically connected with lower and upper shields for allowing the sense current to flow in a direction perpendicular to the sensor plane. A typical CPP TMR read sensor comprises a barrier layer sandwiched between lower and upper sensor stacks. The barrier layer is formed by an electrically insulating nonmagnetic MgOX film having a thickness ranging from 0.4 to 2 nm. When the sense current quantum-jumps across the MgOX barrier layer, changes in the resistance of the CPP TMR read sensor is detected through a TMR effect. A typical CPP GMR read sensor comprises a spacer layer sandwiched between the lower and upper sensor stacks. The spacer layer is formed by an electrically conducting nonmagnetic Cu—O film having a thickness ranging from 1.6 to 4 nm. When the sense current flows across the Cu—O spacer layer, changes in the resistance of the CPP GMR read sensor is detected through a GMR effect.
In a typical TMR read sensor, the lower sensor stack comprises a first seed layer formed by a nonmagnetic Ta film, a second seed layer formed by a nonmagnetic Ru film, a pinning layer formed by an antiferromagnetic Ir—Mn film, and a flux closure structure. The flux closure structure comprises a keeper layer formed by a ferromagnetic Co—Fe film, an antiparallel coupling layer formed by a nonmagnetic Ru film, and a reference layer formed by a ferromagnetic Co—Fe—B film. The upper sensor stack comprises a sense layer formed by a ferromagnetic Co—Fe—B film and a cap layer formed by a nonmagnetic Ru film. Both the Co—Fe—B reference and sense layers exhibit a “soft” amorphous phase after deposition, which will be transformed into a polycrystalline phase after annealing. With this crystallization, a Co—Fe—B(001)[110]//MgOX(001)[100]//Co—Fe—B(001)[110] epitaxial relationship is developed, and thus the TMR effect is substantially enhanced.
In the typical TMR read sensor, four fields are induced and used for proper sensor operation. First, a unidirectional anisotropy field (HUA) is induced by unidirectional antiferromagnetic/ferromagnetic coupling between the pinning and keeper layers. Second, an antiparallel-coupling field (HAPC) is induced by antiparallel ferromagnetic/ferromagnetic coupling across the antiparallel-coupling layer and between the keeper and reference layers. Third, a demagnetizing field (HD) is induced by the net magnetization of the keeper and reference layers. Fourth, a ferromagnetic-coupling field (HF) is induced by ferromagnetic/ferromagnetic coupling across the barrier layer and between the reference and sense layers. To ensure proper sensor operation, HUA and HAPC must be high enough to rigidly pin magnetizations of the keeper and reference layers in opposite transverse directions perpendicular to an air bearing surface (ABS), while HD and HF must be small and balance with each other to orient the magnetization of the sense layer in a longitudinal direction parallel to the ABS.
In order for the TMR read sensor to exhibit a high TMR coefficient (ΔRT/RJ) while maintaining a low HF and a low junction resistance-area product (RJAJ), the lower sensor stack is smoothened for growing a flat barrier layer. Presently, a plasma treatment is applied to the reference layer to smoothen its surface, and then a ferromagnetic film is deposited to replenish the smoothened reference layer. This smoothening technique creates a smooth foundation for the barrier layer to grow with a flat topography, and thus for the TMR read sensor to attain a low HF. However, this smoothening technique inevitably causes argon gas bombardments at a “stitched” interface in the replenished reference layer. These bombardments create contact resistance and deteriorate spin polarization, thus increasing RJAJ and decreasing ΔRT/RJ, respectively.